Variable limit switch assembly



Dec. I5, 1953 G. W. RUSLER, JR, ET AL 2,663,001

VARIABLE LIMIT SWITCH ASSEMBLY Filed OCT.. 13, 1952 /5 fAMPLIFIER a DISCRIMINATOR 83 T/ CLUTCHELEVAToR ALLOWABLE TRM/EI.. LIMITS REFERENGED TO TRIM POSITION FOWARD INDICATED AIR SPEED AFT 0.6.

I I l IDIGATED AIR SPEED INVENTORJ GEORGE w. RusLER JR. By ROBERT c. BURGGREN CENTER GRAVITY ATTORNEY REFERENCED T0 TRIM POSITION ALLOWABLE TRAVEL LIMITS FOWARD Patented Dec. 1,5, 1953 George W. Rusler,

Minneapolis,

Jr., and *Robertv C. Burggren, Minn., assgnors to Minneapolis- Honeywell lRegulator Company,

Minneapolis,

Minn., a corporation of Delaware Application October 13, 1952, Serial No. 314,518

17 Claims. (Cl. 318-489) -servomotor and hence the displacement of the elevator control surface of an aircraft within predetermined limits depending upon the speed of the aircraft is disclosed. In that application it was recognized that the speed of an aircraft had a denite effect upon the required displace ment of the control surface to perform or obtain given attitude displacements of the aircraft. It was recognized that for high airspeeds very small displacements of the controlsurface were required to perform satisfactory controlling operation and conversely wide angle displacements of the control surface were required for given attitude changes at low airspeeds. 'It was recognized that structural limits of the aircraft could easily be exceeded by attempted wide angle displacements of the control surface while the aircraft was traveling at'high speed. The solution in that invention to provide a safety device to Vprohibit the control surface beyond desired displacement limits was to sense airspeed and control a plurality of pairs of limit switches or place the control of the servomotor under a plurality of pairs of limit switches 'depending "upon the airspeed sensed.

In the subject apparatus, only a single pair of limit switches are utilized and these switches are varied in position with respect to one another .depending upon the position of the kcontrol surface of the aircraft.v This apparatus relates speed of travel of the aircraft with any non- .accelerated flight attitude of the aircraft'such as straight and level flight. The allowable travel limits of the control surface for r'safe flight are then determined from a reference position which is a trim position'oria position ofthe control' which averages .the variousf'. positions assun'iedu 55 by the servomotor to'position the switches'in proportion to the average position of the servomotor such that if the servomotor departs rapidly from the Vaverage position and hence beyond limits established for safe operation of the aircraft, one or the other of the limit switches is operated. With the operation of the limit switches, control of the aircraft is'returned to'the operator and hence the apparatusguards against malfunction of the automatic pilot. Y f

It is therefore an'object of this invention to provide an improved flight control apparatus for the aircraft which limits' automatic control with established safe'operating limits. A It is alsoian object of this invention to provide a simplified apparatus for limiting displacement of the control surfaces as a function of airspeed without requiring special equipment to sense-airspeed. Y

A further object of this invention is to provide an improved limit switch assembly operated by a servomotor in an automatic flight control apparatus to protect against malfunction of the ight control apparatus. V v

It is further an object of this'in'vention to provide in anV improved'ight control apparatus a means for discontinuing operation of the limit switchassembly during turns o f` the aircraft.

These and other objects ofthis invention will become apparent from the reading of the attached description together with thedrawings in which: f f Y Figure 1 is a schematic circuit of the improved flight control apparatus;

Figure 2 is a detailed view in section of the improved limit switch assembly of the subject night control apparatus; and Y Figures 3, 4 and 5 area group ofplots of aerodynamic charac'teristlcs oigan aircraft, Figure 3 showing therelationship between allowable travel of the elevator control surface referenced to the trim position of the elevator plotted against indicated airspeed forgiven center of gravities of the aircraft, Figure 1 showing a plot of trim position against indicated airspeed for the various center of gravities ofthe-aircraft and Figure'5, a composite plot of Figures 3 and 4, showing 4allowable travel of the elevator control surface referenced to the streamlined position of the elevator and plottedV against' trim position ofA the elevterw 4 l Our improvediightcontifol apparatus includi'ng 'tlie'variable` limit switclifasse'mblyfof the subject invention appliesto a'utomaticcontrol of the aircraft about"fthe'pitch airis or relative to v Figure 3 `of the drawings.

3 the control of the elevator surface of the aircraft. It is in this axis of flight that the effect of the acceleration of gravity on the mass of the aircraft together with the acceleration imparted to the craft due to manipulation of the elevator control surface often exceed the structural limitationsof the lift surface of the craft, that is the wings. In the'present invention, as will be pointed out as the disclosure proceeds, the speed at which an aircraft is traveling is related to the position of the elevator control surface for given flight attitudes, namely non-accelerated night such as level flight attitude. It has previously been recognized that to avoid overstress of an aircraft of a given gross weight, that the control surface must be kept within allowable defiection limits from a control surface position, known as trim position. These allowable deflection limits are determined by and are a. function of indicated airspeed of the aircraft and the center of gravity locationas will be seen in The trim position of the aircraft for the given gross weight is also a function of indicated airspeed and center of gravity location as shown in Figure 4 of the drawings. As noted above, the data of Figures 3 and 4 provides the plot of allowable elevator travel limits from thetrim position versus the elevator trim position shown in Figure 5 in which the center of gravity'v locations of the before mentioned plots are coincident.

In the present invention and except for turns ing flight attitudes it is assumed that the elevator deflections from trim positions are random, so

l that the short-time averagek of the elevator position will be'the trim position. The limit switches are positioned inaccordance with this short time average which in ,turnv determines the elevator travel limits. Apparatus whose time averaging gives a measure of the trim position positions the switches through differential gearing in accordance with the allowable travel limit plots.

In the present invention our improved flight `control apparatus and limit switch assembly is shown in conjunction, in Figure l, with a schematic disclosure of the electrical circuit for the ',elevator channel of an automatic pilot for an Tat I6, and a servo balance potentiometer or controller indicated at il. i ltiometer includes a resistor 25 and wiper 2|, the

resistor 2!) being connected by conductors 22, 23

The pitch axis potento a pair of centering potentiometers 2d, 25. The

- resistor 25 is energized from a secondary winding 28 of a transformer 29; primary winding 35 of which is connected through a relay indicated at 3| to an alternating current source of power |55.' Wiper 2| of the pitch' axis potentiometer is operated by a vertical gyroscope indicated at 35 through a connection schematically disclosed at 36 connectedto the pitch displacement axis of the gyrosc'ope. Gyroscope 35 is schematically s disclosed in Figure l and since its details form no part of the subject invention they will not be included herein. Servo balance potentiometer Il which includes a resistor 31 and wiper 35 is connected in a bridge type circuit with the pitch axis potentiometer I5, the resistor 31 being con nected by conductors 40, 4| to the centering potentiometers 25, 24 respectively. Also included in the network is va ratio potentiometer 42 which is connected between conductors 40, ill and adjusts the level of the signal inserted in the bridge circuit by the servo balance potentiometer. Wiper 38 of the servo balance potentiometer Il is operatively connected to the output shaft or armature of the servomotor I2 through a con nection indicated at 44 and is grounded at 55 for purposes to be later described. It will thus be seen that the pitch axis potentiometer I5 and servo balance potentiometer form a bridge circuit energized from the secondary winding 28 of transformer 29 with the output of the bridge appearing across the wipers 2| yand 35 of the pitch axis potentiometer and servo balance potentiometers respectively. The wiper 2| of the pitch axis potentiometer is also connected to a center tap 5i! of a resistor 5| of the up-elevator potentiometer I5 through a conductor 52. The up-elevator potentiometer includes a wiper 53 which is connected to the roll axis of the vertical gyroscope through a connection indicated at 55. The extremities of the Winding for resistor 5I of the up-elevator potentiometer are connected by conductors 5S, 51 to a common point 58 and the up-elevator potentiometer is energized from a secondary winding 60 of transformer 29 which connects to the common point 53 and conductor 52 leading to the tap 55 of resistor 5|. Also connected to Wiper 53 is one extremity of a winding 62 of a trimmer potentiometer 83, the

'same sense irregardless of the position to either side of the center tap 50 that the wiper 53 moved. 'I'he specific circuity of the lip-elevator potentiometeris also shown in the above-mentioned co-pending applications or" Benjamin Cisceland Willis Gille and is shown herein only to indicate that in normal automatic pilot operation in the elevator channel of an aircraft, an

. up-elevator signal is supplied to the controlling servomotor upon banking of the aircraft indicating the entry into a turn for the purpose of maintaining altitude in a turn. It will therefore be seen that the output of the bridge circuit formed by the servo balance potentiometer Il and pitch axis potentiometer I5 is added to the output of the up-elevator potentiometer through a trimmer potentiometer 63 or voltage divider which determines what portion of the `rlp-elevator signal will be added in series with the before mentioned bridge circuit. The output of this network is connected by a conductor it which leads from the wiper 64 of the trimmer potentiometer 63 to the input stage of a discriminator amplifier shown schematically in the drawing at This discriminating type amplifier is shown schematically herein inasmuch as its details form no part of the subject invention and reference may be had to the Gille et al. Patent 2,425,733,

dated August 19, 1947, for a disclosure of the same. The input stage ofV this discriminating amplifierr also includes a grounded terminal indicated at 1I which grounded terminal completes with grounded terminal 45 the electrical circuit through the network Ill. The discriminating ampliier il also includes power terminals indicated schematically at 'I2 and apair of discriminating relays i3, 'I4 including contact mechanisms which control the application of D. C. power from a source indicated at l5 to one or the other of the directional eld windings le, l'I of the servomotor I2 and through the armature of the servomotor to a grounded connection indicated at 8l) through a circuit to be more fully described above. .Y

The D. C. power supply 'i 5 indicated herein as a battery source, and grounded at 8|, feeds the power or energizing circuits for the automatic pilot through an engage relay indicated at 19. The engage relay includes energizing coil 82V and a pair oi contacts 83, 84 connected in series with the relay contacts 13, 74 o1" the discriminator amplier and the field windings l5, 7l of the servomotor. The power circuit also includes a momentary disengage switch B5 and a momentary enegage switch 85 as well as a holding cone tact It on relay 'i3 and a contact ISA to control the energization of a clutch 9i connecting the elevator or control surface of the aircraft to the armature or output shaft of the servomotor i2. 'Ihe energizing circuit for the engage relay is as follows: battery 'I5 to normally closed disengage switch 8S, conductor Si to a limit switch contacts 93, Q4, conductor 38 to normally open mou mentary engage switch 35, to coil 62 and a ground connection returning through the ground connection 8| of battery l5. Closure oi the momentary engage switch 85 energizes the relay coil S2 and with the closure or the contacts, the holding Contact Ille is closed setting up a holding circuit around the momentary switchi in a well known manner. The clutch si will simultaneously be energized from the plus side of the battery through conductor 8S and contact H34 .f

to av conductor 03 and clutch SI and from the grounded side of the clutch to the grounded terminal 8| of the battery. Further, closure oi' the relay 'it will operate or close the contacts t3 i4 connecting the plus side of the D. C. supply through a conductor Ils, contacts 83. to the contacts of the discriminating reiay la,r it. Thereafter and depending upon the operation of the discriminating amplifier one or the other of the relays i3, 'is will become operative to energize one or the other o the field coils l5, 'il and the armature or the servomotor i2. It will be seen that the power of D. C. energizing circuit for the automatic pilot may be deenergized by opening the momentary switch Bt deenergiaing relay l@ or by operation of one or the other of the contacts S3, Se of the limit switch assembly and thereby disengaging the relay as well as eenergizing clutch Si and the servcmotor as well as other components of the automatic pilot energized from the D. C. supply and not shown herein.

The operation ci servomotor I?. causes rotation of the output shaft to displace the control surface through the clutch il! and also moves .the servo balance pot wiper 3s through shaft Il connected to the output shaft. An extension of the shaft 46, indicated generally at operates as will be later noted the contacts Se of a variable limit switch assembly 92. In Figure 1 the limit switch assembly is shown in blockform only, the details being disclosed in Figure 2. The only electrical components-of the variable limit lswitch assembly Shownin Figure 1 are the switches 93, 94 and a solenoid coil S5. The solenoid coil is a braking solenoid of the limit switch assembly which will be more fully described below. This coil is electrically independent from the automatic pilot except for deriving its energization from the D. C. source l5. Energization of the braking solenoid whose operation will be explained hereinafter is controlled by a switching device |28 mounted on the output axis lill of a conventional yaw rate gyroscope shown schematically at IBS which when the aircraft is turned will'precess against the opera-tion of restraining springs |06 to operate the switching device |03 and connect the braking solenoid to D. C. supply 15. The electrical circuit for this phase of the apparatus is as follows: plus side of battery "l5, conductor IIE, coil Q5, conductor I I2, to movable contact H3 of a switching device connected to the output axis of the yaw rate gyroscope IHS, to one or the other of a pair of elec trically connected stationary contacts Ilfl of the switching device IUS, to ground connection I and from ground 8l. to battery l5. It will be seen that regardless of the direction of turn of the aircraft, displacement of the gyroscope will Vcause one or the otherof the stationary contacts II4 of device i538 to be engaged by movable contact II3 and provide the same energizing circuit for the braking solenoid coil 95.

In addition to the power control circuit referred to above, the automatic pilot equipment includes a control circuit engage switch or relay 3i which disconnects or connects the alternating power source |00 to the primary winding 3s of the control transformers. Engage relay 3| includes a coil which is connected across the supply |00 by a conductor |02 and a conductor IOI to a manually operated switch Sl and a conductor 98. The manually operated switch Si when moved in the closed direction, operates to close the energizing circuit for the relay coil S6 operating the contacts of the relay 3| to connect the alternating source of .power to the primary windingV .33. This engage relay and control power circuit is not aected by disengage safety devicessuch as the momentary switch 86 and the limit switch contacts s3, 94 inasmuch as it controls merely the application of power to the control circuit and the amplier and it is normally desired to maintain the control circuit in standby opera-tion even though the autopilot is not engaged. Thus disengagement is eiected merely by control through the power or energizing circuit for-the autopilot controlling equipment such as the servomotors. Thus it will he seen that the limit switch assembly does not operate to disengage power to the controlling portions or the autopilot network and the ampliiier:

The limit switch assembly 92 is shown in detail in Figure 2, and it will be seen that it is adapted to bemounted on a portion of the housing of the servomotor indicated generally at |26 common'tothe servo balance potentiometer asse-; bly I'I to be operated by the shaft 44 which operates the servo balance potentiometer. In liigureV 2 it will be seen that the wiper 33 of the servo balance potentiometer I'I is carried on the shaft 44 through suitable clamping Vmeans and the associatedresistor'winding 3l is mounted in a rflange or bracketI indicated at I2I and having 7 a conventional slip ring arrangement |24 associated therewith. Concentric with the shaft 44 and attached to the khousing |20 by suitable screws and positioned by pins |22 is a housing |23 for the limit switch assembly. Positioned within this housing |23 is the shaft 90 which acts as an extension of the shaft 44 being connected thereto by suitable means such as splining so that it will be driven by the rotation of shaft 44. Mounted adjacent the wiper 38 on shaft 44 and secured thereto by suitable means such as a lock nut |25 is an arm |26 having a pin extremity |21 extending normal thereto. The pin |21 cooperates with one or the other of the flanged extremities of a pair of spring drive members |30, |3| which enclose and are attached to respectively the ends of a spiral spring indicated at |32. The spring drive member |3| as will be seen in the drawing is attached to a sleeve |33 concentric with the shaft 90 and mounting one extremity of the spiral spring |32 through a pin indicated at |35. The sleeve |33 is journalled or loosely mounted on the shaft 90 and it in turn mounts the drive member |30 through a sleeve type mounting such that the parts may rotate relative to one another. Therdrive member |30 attaches to the opposite extremity of the spiral spring, not shown in the drawing. Also Vcooperating with the flanged portions of the spring drive members |30, |3|, only the ange of the drive member |30 being shown in the drawing, is a pin |38 which is mounted on a circular plate |30 having a toothed periphery |4|. It will be understood from the drawing that the arm |26 is driven by the shaft 44 and hence urges the pin |21 mounted thereon into engagement with one or the other of the spring drive arms |30, |3| additionally tensioning the spring |32. It will be noted that pins |38 and |21 are placed at different radial distances from the shaft about which they are concentrically located and consequently do not interfere with one another. Depending upon which of the spring arms |30, |3| are engaged by pin |21, the opposite arm will in response to the tensioning of the spring |32 engage the pin |38 and drive the same in the direction of rotation of the pin |21. The pin |38 being attached to the geared plate |39 will cause the plate |39 to be driven in response to movement of the pin |38. Plate |33 is mounted on a sleeve |40 which is journalled through suitable means such as sleeve type bearings |45 on the shaft 90 and mounts a paddle |42 of a viscous damper. The viscous damper includes a stationary housing |43 with an opening |44 or cavity therein within which the paddle |42 is located and within which opening a viscous medium or fluid is contained, the casing |43 being attached to an end wall portion |50 of the casing inember |23 of the limit switch assembly. Suitable sleeve type bearings |46 are positioned between sleeve |40 and housing |43 to provide for relative movement therebetween. Screw means indicated at secure the casing to the end wall |50. The casing includes suitable sealing members such as gaskets indicated at |53 which prevent leakage of the uid from the cavity |44 of the casing. Also included in the casing is a plugged aperture |54 through which the fluid medium is inserted. Inasmuch as the viscosity of the fluid medium might be apt to change over varied temperature ambient, a heater element indicated only in phantom at |56 and a thermostat indicated in phantom at |51 can be mounted on the casing |43 to maintain the ambient o f the fluid within a predetermined range. Inasmuch as the heater and temperature controller and the associated energizing circuit are required only for wide ambient temperature changes and their details are not of importance herein to the operation of the limit switch assembly, these elements are shown only in phantom. The extremity of sleeve |40 remote from the plate |39 mounts a gear member |60 which is mounted and secured thereon through suitable clamping means indicated at |6|. Gear |60 cooperates with a pinion |62 mounted on a shaft |63 which is journalled in suitable bearing means |64 mounted in end plate |50 at one extremity and in a supporting plate |65, the mounting of which will be later described. Cooperating with pinion |62 is a gear |10 mounted on a shaft |1| which is also journalled between the end plate |50 of casing |23 and supporting plate |65 through suitable means not shown. Included on shaft |1| is a pinion |12 driven thereby which cooperates with a gear |14 mounted on a sleeve |15 which is supported by and journalled on sleeve |40 at one extremity by a bearing |16 and on shaft 95 by a bearing |11 respectively. Gear |14 is mounted on the sleeve |15 through suitable means not shown and the opposite extremity of the sleeve carries a switch mounting arm |19 carrying the switch 04 at the extremity thereof, being retained thereon by a bracket |80, the mounting arm being secured to the sleeve by a suitable clamping indicated at |8|. Thus it will be seen that rotation of gear |60 will cause pinion |62 to drive the gear |10 and hence pinion |12 in a particular direction, the pinion |12 driving gear |14 and hence the sleeve |15 and switch mounting arm |19.

Also mounted on shaft |63 isa gear |82 which cooperates with a gear member |83 mounted on a second sleeve |84 which is journalled on the sleeve |15 through suitable sleeve type bearing means indicated at |86. Gear |63 is clamped to the sleeve |84 through suitable clamp nut means indicated at |81. The extremity of sleeve |34 opposite the mounting of gear |63 mounts a switch arm |88 carrying a switch 93 at the extremity thereof, the switch being carried on arm |63 by a bracket |90 and being mounted on the sleeve |84 through suitable means not shown. It will be seen that rotation of the gear |60 driving pinion |62 will also drive a separate gear train from that cooperating with the switch arm |19 and switch 94 in the form of a drive starting with gear |82 through gear |33 and sleeve |84 to switch arm |38 and switch 93. Also clamped on the extremity of shaft 90 being held in position by a nut |92 is a cam member |98 which cooperates with either of the switches 94 and 93 driven through the gearing described above. The details of these switches are not included herein inasmuch as they form no part of the present invention but it is understood that a suitable switch such as a microswitch unit may be used. Similarly the details of the electrical connections to the switches are not of importance except that they are included in the circuit in the manner described in connection with Figure 1. Plate |65 which serves to support the shafts |63 and |1| of the gear train is joined on sleeve |84 through suitable ball type journalling means indicated at |95 and is supported on housing |23 through extensions |96. Enclosing the gearing and switches is a suitable removable cover indicated at 200 which lits over a flanged portion 20| of the frame |23 of the variable limit switch assembly. Also included on the surface of the casing |23 is a solenoid 202, the winding or coil 95 of which has been previously discussed. This solenoid includes a movable plunger portion 203 attached to a shaft 204 having a braking or toothed extremity 205 adapted to engage the toothed periphery or toothed portion MI of platel39. The shaft 204 is biased through a spring 205 to a position where the brake shoe or toothed shoe 205 is out of engagement with the teeth I4| of plate |39. Upon energization of the solenoid, the spring 206 is compressed and the brake shoe 205 engages a toothed periphery IM of plate I39 to rigidly hold the same in a fixed position.

It will be seen that the limit switch assembly uponcrotation of shaft M causes the arm |26 to engage one or the other of the spring drive arms Iil, IBI and through the tension applied to spring |32 drive the pin |38 and hence the plate |39 in a direction to follow the rotation of the shaft. Rotation of the plate |39 causes sleeve It@ to rotate the paddle |42 of the viscous damper within the iiuid contained in the casing HIE of the damper at a rate determined by the viscosity of the'fluid and the tensioning cf the spring |32. Assuming temperature conditions to be constant, the rotation of the sleeve Hit will be an average of the rotation of the servomotor. Its rotation will be dependent upon speed and the amount of rotation of the shaft M and the clamping of the viscous damper. Continuing now with the gear drive it will be seen that the rotation ofthe sleeve |40 with the paddle 52 thereon will cause the gear trains described above to rotate urging switches 93, 94 in opposite directions of rotation, one rotating in the saine direction as the cam |95 mounted on the extremity of the shaft 90 splined to shaft 44 and the other in an opposed direction. As will be pointed out more fully hereinafter, the ratio of the various gears or gear teeth in the two gear trains is such as to cause the switches to be brought closer together or the spacing between the switches to be diminished as the shaft 44 rotates in one direction and to move the switches vapart as the shaft rotates in the opposite direction. The ratio of the gearing is such as to pro vide rotation of the switches S3, 94 respectively so that they will follow particular curves or relationships with respect to the position of the output shaft of the servomotor and hence the shafts and 90 with the cam |95 thereon. It will also be seen that if the shaft 44 and hence the shaft et and the cam |95 driven thereby is rotated beyond the average position such as being rotated rapidly for a given period of time or through a given angular distance, one or the other of the switches et, willfbe engaged.

Operation As pointed out above,` the details of the network Iii, amplifier II, and servomotor I2 are largely conventional in the design of an automatic pilot for an aircraft. The present invention is directed to van improved automatic pilot system including ja limit switch assembly designed primarily asa safety device to be used with the automatic pilot in the event of malfunction of the automatic pilot. The actual functioning of the various parts of the variable limit switch assembly 92 has been included above in connection withv the Ydescription of the parts thereof for the purpose of better understanding the relationship" betweenv those parts.

Referring now to Figure 1 it should be noted that the operation of the network I0, amplifier I I and servo I2 making up the automatic pilot system is conventional with that described in connection with the above-named Ciscel application and the Gille patent on the disclosure of the amplifier II. rihe subject limit switch assembly is normally not operated except in the event of malfunction of the automatic inasmuch as operation of the servomotor will generally cause an attitude change in the aircraft sufficient to compensate for the displacement sensed by the controller or vertical gyroscope 35 and thereby deenergize the servomotor I2 before the limit switches are approached. Under automatic control, that is with both control and power energizing circuits energized the pitch axis of the vertical gyroscope 35 controls the pitch axis potentiometer and any displacement from apredetermined attitude for which the gyroscope has been set will be sensed by the pitch axis potentiometer I5 causing unbalance in the network I0 and energizing the amplifier I I. Depending upon the sense of the output signal of the network I0 one or the other of the relays 13, 14 will be operated to energize one or the other of the directional field windings 16, I1 of servomotor I2 together with its armature through the circuit described above to cause operation of the servomotor and rotation of the control surface to bring the aircraft to an attitude wherein the gyroscope reaches its set or predetermined position thereby bringing the wiper 2I of the pitch axis potentiometer back to its original position with respect to the winding 20 of the potentiometer. Simultaneous with the operation of the servomotor is displacement of the wiper 38 of the servo balance potentiometer which tends to offset or counteract the signal output of the pitch axis potentiometer and hence prevent overshooting or over-control by the autopilot. As previously indicated the ratio and centering potentiometers are adjusted prior to flight to establish a null output from the network I0 when the wipers of the servo balance potentiometer I'.' and pitch axis potentiometer I5 are centered on their respective windings. Also the ratio potentiometer determines the effect of the servo balance potentiometer in counteracting the output signal from the pitch axis potentiometer. The up-elevator potentiometer I6 is included in the network and operated from the roll axisv of the vertical gyroscope to introduce an up-elevator signal into -the network I0 upon turning of the aircraft. function is designed to offset the normal loss of altitude of an aircraftin turns. The trimmer potentiometer 64 associated therewith determines' the amount of the up-elevator signal to'be' fed to the amplifier II. Also in turns, the yawtrate gyro |09 becomes effective to operate or energize the solenoid of the limit switch assembly locking the plate |39 with respect to the casing |23 and consequently precluding movement of the limit switches 93, 94. As was pointed out above, an up-elevator signal is generally inserted into the autopilot upon turning of the aircraft and in order to compensate or counteract for the false indication of this up-elevator signal withV respect to the limit switches, the solenoid is operated to hold the limit switches with the particular spacing between the switches which existed when the aircraft entered into the turn.v The introduction of up-elevator signal and the movement of the control surface in an yup-elevator direction would normally lcause the spacing between the ThisA iiini'i switches in widen somewhat indicating a slower flight and since this is not necessarily the casein turns. the switches are locked in the position in which theywere when the aircraft entered the turn. It has been found in certain instances that this phase of the operation is not necessary in some aircraft and hence the solenoid locking provisions of the limit switches in turns together with the yaw rate gyro controlling the vsame can beeliminated,

.Upon operation of the servomotor, as indicated above,` the rotation ofthe output shaft of the servornotor causes the shaft 90 of the limit switch assembly to Yrotate the 'cam |98l at the rate of rotation or output of thel servomotor. At the saine timeithe arm |26 will be driven from the shaft 44 causing one or the other of the arms (3,0, |3I to Aadditionally tension the spring |32 and move the spring drive arm not engaged by the alfrn |26 to follow and drivingy the plate |39 arid hence the viscous damper paddle l|42 at a rate determined by the viscosity of the fluid in the damper to time average the movement of shaft 44. The rotationof the paddle |42 and hence the shaft |40, will drive the gears |60, |62, |10, i12.; and |14 to move switch mounting arm H8 and Vswitch 9.4 in the direction opposite to the rotation of the shaft so and io a, position in ii cated by the relationship of one of the allowable travel limit curves of Figure with respect to the trim position. Similarly the switch arm |88 and switch 93, will be driven bythe gearing |60, |52, isz vand |83 to the vposition indicated by une other allowable travel 'limit curve of Fig-ure 5 referenced to. Vthe. trim position and in the same direction of rotation as the output shaft. In the event that the rotation of the output shaft exceeds therotation of one or the other of the switches 93, 94 with respect to the shaft '90, one or the other of the switches will be operated to open, deenergizing the engage relay'w causing the clutch 9| andservomotor l2 tobe deener- Sized-end 4the aircraft returned to manualcontrol withthecontrol Circuit energized and operative but not eeotive Yto'fco'ntrol servomotor l2. Itv will also befseen thatwhether thecontrol surface 1S in an. uli-elevator or downeelevator position. thatAoperation-of the` servomotor in either direction from the trim position will cause operation: ofthe spring drive and viscous damper to time average the servomotor rotation and p1osi tion the switches inthe saine manner but with differing spacings relative to one' another.

. Inconsidering this invention, it should be kept in mindthatthepresent*disclosure is intended to be illustrative only andthat fthe scope'of the invention is to be determined by the appended claims. u Y y We claim as our invention:

1. Apparatus for controlling a Vservomotorop- Gratins a control surface of an aircraft to maintain a predetermined condition ofrflight of the aircraft, comprising, a servomotor for lcontr'olli'ing vthe position of said control surface of said aircraft.- power circuit meansrfor energizing said servomotor. a. control circuit means operative upon an indication of'a need for operating lSaid servomotor to maintainthe predetermined condition of flight of the aircraft for controlling the operation of said power circuit means. limit switch means includingcam means and al pairof limit; switches adapted to be operated thereby, means Connecting saidcam means to said servomotor .to be operated directly thereby, means drivingly connecting said limit switches to said servomotor including a spring and viscous damper with different ratio gearing driving each of said switches, said gearing rotating said switches rela-- tive to one another and to said cam in proportion to the allowable travel limits of said control surface for said predetermined condition of flight of the aircraft, said limit vswitch means being adapted to further control the operation of said power circuit means.

2. Apparatus Afor controlling a servomotor operating a control surface of an aircraft to maintain apredetermined condition of flight of the aircraft, comprising, aservomotor for controlling the position of said control surface of said aire craft, power circuit meansl for energizing said servomotor, a control circuit means operative upon an indication of a need for operating said servomotor to maintain the predetermined. condition of flight of the aircraft for controlling the operation of saidA power circuit means, limit switch means including cam means and a pair of limit switches adapted to be operated thereby, means connecting said cam means to said servomotor to be operated directly thereby, means drivingly connecting said limit switches to said servomotor including a spring and viscous damper with different ratio gearing driving each of said switches, said gearing rotating said switches relative to one another and to said cam in proportion to the allowable travel limits of said control surface for said predetermined condition of flight of the aircraft, said limit switch means being adapted to further control the operation of said power circuit means, and means responsive to turning of said aircraft for holding said limit switches independent of the rotation of said servomotor.

3. Apparatus for controlling a servomotor operating a control surface of an aircraft to maintain a predetermined condition of fiight of the aircraft, a servomotor adapted to be connected to the control surface of the aircraft, power circuit means for energizing said servomotor, a control circuit means operative upon an indication `of a need for operation of said servomotor to maintain the predetermined condition of ight of the aircraft for controlling the operation of said power circuit means, a pair of limiting devices and operating means therefor, means connecting said limiting devices and said operating means to said servomotor such that said limiting devices will have a differential therebetween in proportion to a' predetermined set of allowable travel limits for said control surface of the aircraft corresponding to the time `averaged position of said control surface of the aircraft at that instant and so that` the operating means will be in position to cooperate with one of said limiting devices, and means included in said connection means for permitting said operating means to engage one of said limiting devices if the instantaneous displacement of control surface of said aircraft exceeds the time 'averaged position of the control surface said limiting devices further controlling the operation of said power circuit means.

4.v Apparatus for controlling a servomotor operating a control surface of an aircraft to maintain a condition of iiight of the aircraft, a servof motor adapted to be connected to the control surface of the aircraft, power circuit means for energizing said servomotor, control circuit means operative upon an indication of a need for opera tion of said servomotor to maintain the predetermined condition of flight of the aircraft for cona pair of limiting devices and an operating means therefor, means connecting theroperating ymeans of said limiting devices to said servomotor, a ilexible connection connected to said servomotor and `transmitting movement of said servomotor in proportion to its speed of rotation and elapsed time `of rotation, gearing means connecting said flexible connection and said limiting devices to rotate one said limiting devices in the same direction of rotation as said servomotor and one in ythe opposite direction of rotation from said servomotor and varying the spacing between the limitling devices to cause the limiting devices to approach one another for one direction of rotation of the servomotor and to depart from one another with an opposite direction of rotation of the servomotor, and means included in said power circuit means and operated by one or the other of said limiting devices for further controlling the operation of said power' circuit means.

5. Apparatus for controlling a servomotor operating a control surface of an aircraft to main tain a condition of flight of the aircraft, a servomotor adapted to be connected to the control surface of the aircraft, power circuit means for energiizng said servomotor, control circuit means operative upon an indication of a need for operation of said servomotor to maintain the predetermined condition of flight of the aircraft for controlling the operation of said power circuit means, a pair of limiting devices and an operating means therefor, means connecting the operating means of said limiting devices to said servomotor, a flexible connection connected to said servomotor and time averaging the movement of said servomotor, gearing means connecting said flexible conection and said limiting devices to rotate on said limiting devices in the same direction of rotation as the servomotor and the other of said limiting devices in an opposite direction of rotation and varying the spacing between the limiting devices to cause the limiting devices to approach one another for one direction of rotation of the servomotor and to depart from one another with an opposite direction of rotation of the servomotor, means included in said power circuit means and operated by one or the other of said limiting devices for further controlling the operation of power circuit means, and means responsive to turning of said aircraft for locking said limiting devices.

6. Apparatus for controlling a servomotor operating a control surface of an aircraft to maintain a predetermined condition of flight of the aircraft comprising power circuit means for energizing said servomotor, control circuit means connected to said power circuit means and operative upon an indication of a need for operation of said servomotor to maintain the predetermined con- .dition of flight of the aircraft for controlling the operation of said power circuit means, a limit switch assembly including a plurality of relatively movable parts two of which cooperate with a third ,part to perform a switching operation for either direction of rotation of the servomotor, a differential gearing means connected to said two parts of said limit switch assembly to rotate said parts in the different directions of rotation and varying the spacing between said parts to cause them to approach one another for one direction of rotation of the servomotor and to depart from one another with an opposite direction of rotation of the servomotor, flexible connection means connecting said gearing and said servomotor to transmit an average of the movement of said servomotor to said gearingand said twoparts of said limit switch assembly, further means con necting said first part of said limit switch assembly to said servomotor to be directly operated thereby, and means connecting said limit switch assembly to said power circuit means such that it will be further operated by said limit switch assembly.

7.7Apparatus for controlling the flight of an aircraft comprising, a servomotor adapted to be connected to a control surface of an aircraft to maintain a predetermined condition flight of the aircraft, power circuit means for energizing said servomotor, control circuit means for controlling the operation of said power circuit means, means included in said control ciruit means and operative upon an indication of a need for operation of said servomotor to maintain the predetermined condition of flight of the aircraft for controlling the output of said control circuit means, a variable limit switch assembly for further controlling the operation of said power circuit means and including a pair of relatively movable parts and a cooperating operating means for said parts, a slowly operative motor means controlled by the operation of said servomotor, differential gearing driven by said slowly operative motor means and connected to said pair of parts of said limiting assembly for varying the differential between said parts depending upon the direction of rotation of said servomotor, and means connecting said operating means to said servomotor such that it will operate one of said parts when the rotation of the servomotor exceeds that of said slowly operated motor.

8. Apparatus for controlling the flight of an aircraft comprising, a servomotor adapted to be connected to a control surface of an aircraft to maintain a predetermined condition of flight of the aircraft, power circuit means for energizing said servomotor, control circuit means for controlling the operation of said power circuit means, means included in said control circuit means and operative upon an indication of a need for operation of said servomotor to maintain the predetermined condition of flight of the aircraft for controlling the output of said control circuit means, a varaible limit switch assembly for further controlling the operation of said power circuit means and including a pair of relatively movable parts and a cooperating operating means for said parts, a slowly operative motor` means controlled by the operation of said servomotor, and operative to average the rotation of said servomotor, differential gearing driven by said slowly operative motor means and connected to said pair of parts of said limiting assembly for varying the differential between said parts depending upon the direction of rotation of said servomotor, means connecting said operating means to said servomotor such that it will operate one of said parts when the rotation of the servomotor exceeds that of said slowly operated motor, and means operative to brake said slowly operative motor means upon turning of said aircraft.

9. Apparatus for controlling the flight of an aircraft comprising, a servomotor adapted to be connected to a control surface of the aircraft to maintain a predetermined condition of ight of the aircraft, power circuit means for energizing said servomotor, control circuit means for controlling the operation of said power circuit means, means included in said control circuit means and operative upon an indication of a need for operation of said servomotor tomaintain the.

accenni predetermined condition of flight of the aircraft for controlling the output of said control `circuit means, a slowly operated motor -m-eans controlled by the operation of said servomotor, a variable limit switch assembly including a pair of relatively movable parts and a third part,- a dini-erential gearing connecting said relatively movable parts of said limit switch assembly tc said slowly operated motor means for positioning the same such that the spacing between said parts `will be proportional to a set voi allowable travel limits of the control surface of the aircraft for the time averaged position of the control surface of the aircraft at that instant, means connecting said third part to said servomotor to cooperate with said pair of relatively movable par-ts such that said limit switch assembly will be operated if the rotation of the servo-motor exceeds the time averaged position of the control surface of the aircraft, and means operated by said limit switch assembly :for deenergizing sai-d power circuit means.

l0. In combination, witha server-motor, an output shaft, switch operating means mounted on said output shaft, a first lever connected to and driven by said output shaft, spring drive 'assembly journalled on said output shaft and `adapted to vbe engaged a-nd operated -by said lever, Va second lever adapted to be engaged and dri-ven by said spring drive assembly, a viscous damper including a pair Yof relatively movable parts, said second lever being connected to one 4of said relatively movable parts of said viscous damper, a diiierential gear train assembly including a pair of gear Atrains connected to said one of said relatively movable parts of said viscous dam-per, a pair of switches connected respectively to said pair -of gear trains and positioned concentric with said output shaft and said switch operating means, said gear trains being designed to rotate one of said switches in the same direction of rotation as said `servo -output shaft upon rotation of lsaid Aservo shaft and the other in the opposite direction from sai-d servomotor and varying the spacing between said switches to cause said switches 'to approach one another for one direction of rotation of -Said shaft and to depart from one another in the cpposite ydirection of rotation of said output shaft, and casing means enclosing saidoutput shaft and said switch and mounting said gea-ring and said lother part -of said vis-A cous damper, said casi-ng means -being 4attache to said servomoton l1. In combination `with -a servomotor, an output shaft, vswitch operating means vmounted on said output shaft, a -rst `'lever connected to and driven by said output shaft, a spring -drive assembly journal-led -on said output shaft and -connected adapted to Ybe engaged and opera-ted by said lever, a second =lever adapted to be engaged and operated by spring -drive assembly, a viscous damper including a pair of relatively' 4moe/"able parts, said second llever being -connected to one of said relatively movable parts of said viscous damper, a differential gear train assembly lincluding a pair -of ygear trains connected to said one of said relatively movable parts kof said viscous fdainper, a pair .of :switches connected respectively to said pair of gear :trains :and :positioned 4concentric `rwith said output shaft-and said switch operating emeans, said gea-r trains being designed 4to rotate said switches upon rotation yof saidservoshaft varying the spacing between said switches to cause said switches to approach one another .tor -one direction .of irotation of said shaft and to depart from one another in the opposite direction of rotation .of said output shaft, casing means enclosing said output shaft and said switches and mounting said gearing and said other part of said viscous damper, said casing means being attached to said servomotor, and solenoid means mounted on said casing and including a brake means operative to arrest move.-y ment of said switches.

12. Apparatus for controlling the operation of a servomotor, an energizing circuit means for said servomotor, con-trol circuit means for controlling the operation of said Ienergizing circuit means, means included in said -control circuit means and operative upon an indication of a need for operation of said servomotor for .controlling the Operation of said control circuit means, a variable limit switch assembly adapted to be mounted on and operated by said servomotor including a pair of relatively movable lparts and a cooperating oper-ating means for said par-ts, a slowly operative moto-r means controlled by the operation of said servornotor, differential motion transmission vmeans rdriven by said slowly operative motor means and connected to said pair -o-f parts of said limit switch assembly for varying the differential between said parts depending upon the directionoi rotation -of said servomotor, means connecting said `operating means to said servomotor such that it will operate one of said .parts when the rotation of said servo-'- motor exceeds that -of said slowly :operated motor, and means connecting said variable limit switch assembly `to said energizing circuit means for said serv-omotor to further 4control the operation of said energizing circuit means.

13. Apparatus for controlling the operation -of a servomotor, an energizing circuit means for said servomotor, vcontrol circuit means for -controlling the 4o eration -of said energizing circuit means, means included in said control circuit means and operative upon an indication of `a need ior operation of said servomotor for controlling the oper-ation of said control circuit means, a variable limit switch assembly adapted to -be mounted on and operated :by said servomotor including a pair of relatively movable parte and a cooperating oper-ating means for said parts, a slow-ly operative lmotor means controlled by the operation of said -servomotor -for time averaging the rotation of said servomotor, differential gearing driven by said slowly operative motor means and Iconi-rooted xto said pair of parts of said ilim-it switch assembly for varying the dierential between said parts depending upon the direction of rotation of said servomotor, means connecting said operating means to said 'servomotor such that it -wil-l operate one of said parts when the rotation of said servomotor exceeds that of -said slowlyoperated motor, means `connecting said variable 'limit switch assembly to said energizing circuit means for said servomotor to Afurther control the operation of said energizing circuit means, and `means operative to brake said slowly loperative motor means.

14. Apparatus `for controlling the operation of a servomotor comprising with a servomotor, an

energizing .circuit means for said servomotor,

control circuit means lfor controlling the loperation -of said energizing circuit means, Ameans included in said control circuit means and operative upon a need :for .operation of the servomotor for .controlling .the output of said control lcircuit means, a variable limit :switch assembly vfor dursther controlling the operation of said energizing circuit means and including a pair of relatively movable parts and a third part, a differential gearing connected to said relatively movable parts of said limit switch assembly, a slowly operated motor controlled by the operation of said servomotor, means connecting said differential gearing to said slowly operated motor means to position said diierential gearing and said relatively movable parts of said limit switch such that the spacing between said relatively movable parts of said limit switch assembly will vary for different directions of rotation of said servomotor, means connecting said third part to said servomotor to cooperate with said pair of relatively movable parts such that said limit switch assembly will be operated if the rotation of the servomotor exceeds the rotation of said slowly operated motor means.

l5. In combination with a servomotor, an output shaft connected to said servomotor, switch operating means mounted on said output shaft, a second shaft connected to said output shaft, a slowly operative motor means connected to and driven by said second shaft for time averaging the rotation of said second shaft, a differential gear train assembly including a pair of gear trains connected to said slowly operative motor means, a pair of switches connected respectively to said pair of gear trains and positioned concentric with said switch operating means, said gear trains being designed to rotate one of said switches in the same direction of rotation as said switch operating means upon rotation of said servomotor and the other of said switches in the opposite direction from said switch operating means and varying the spacing between said switches to cause said switches to approach one another for one direction of rotation of said servomotor and to depart from one another in the opposite direction of rotation of said servomotor, and casing means enclosing said output shaft and said switches together with said slowly operative motor means, said casing means being attached to said servomotor.

16. In combination with a servomotor, an output shaft connected to said servomotor, switch operating means mounted on said output shaft, a second shaft connected to said output shaft, a slowly operative motor means connected to and driven by said second shaft for integrating rotation of said second shaft, a diierential gear train assembly including a pair of gear trains connected to said slowly operative motor means,

a pair of switches connected respectively to said pair of gear trains and positioned concentric with said switch operating means, said gear trains being designed to rotate said switches upon rotation of said servomotor varying the spacing between said switches to cause said switches to approach one another for one direction of rotation of said servomotor and to depart from one another in the opposite direction of rotation of said servomotor, casing means enclosing said output shaft and said switches together with said slowly operative motor means, said casing means being attached to said servomotor, and solenoid means mounted on said casing including a brake means effective to arrest the movement of said slowly operative motor means.

17. In flight control apparatus, a servomotor adapted to be connected to a control surface of an aircraft to maintain a condition of flight of the aircraft, power circuit means for energizing said servomotor, a control circuit means for controlling the operation of said power circuit means, said control circuit means including means operative upon an indication for need of operation of said servomotor to control the operation of said control circuit means, a variable limit switch assembly including a pair of limit switches and an operating means therefor, means connecting said operating means to said servomotor for operation thereby, a differential gearing connected to the said limit switch devices and varying the angular spacing therebetween depending upon the direction and amount of rotation of said gearing, a slowly operative means connecting said servomotor and said differential gearing and operated by said servomotor for positioning said limit switches relative to said operating means in proportion to the average position of said operating means, said limit switches becoming operative when the rotation of the servomotor exceeds that of said slowly operative means, and means energizing said power circuit means and controlled by the operation of said limit switches.

GEORGE W. RUSL-ER, JR. ROBERT C. BURGGREN.

References Cited in the le of this patent UNITED STATES PATENTS Number Name Date 2,442,711 Schaedler June 1, 1948 2,615,154 Wade Oct. 21, 1952 

